Vacancy-assisted domain-growth in asymmetric binary alloys: a Monte Carlo study

A Monte Carlo simulation study of the vacancy-assisted domain-growth in asymmetric binary alloys is presented. The system is modeled using a three-state ABV Hamiltonian which includes an asymmetry term, not considered in previous works. Our simulated system is a stoichiometric two-dimensional binary alloy with a single vacancy which evolves according to the vacancy-atom exchange mechanism. We obtain that, compared to the symmetric case, the ordering process slows down dramatically. Concerning the asymptotic behavior it is algebraic and characterized by the Allen-Cahn growth exponent x=1/2. The late stages of the evolution are preceded by a transient regime strongly affected by both the temperature and the degree of asymmetry of the alloy. The results are discussed and compared to those obtained for the symmetric case.Comment: 21 pages, 9 figures, accepted for publication in Phys. Rev.

Effect of the vacancy interaction on antiphase domain-growth in a two-dimensional binary alloy

The influence of diffusing vacancies on the antiphase domain growth process in a binary alloy is studied by Monte Carlo simulations. The system is modelled by means of a Blume-Emery-Griffiths hamiltonian with a biquadratic coupling parameter K controlling the microscopic interactions between vacancies. We obtain that, independently of K, the vacancies exhibit a tendency to concentrate on the antiphase boundaries. This gives rise to an effective interactions between movin interfaces and diffusing vacancies which strongly influences the domain growth process. One distinguishes three different behaviours: i) for K 1 the growth is slown down but still curvature driven

Monte Carlo study of the domain growth in nonstoichiometric two-dimensional binary alloys

We use a nearest-neighbor antiferromagnetic Ising model with spin-exchange dynamics to study by Monte Carlo simulations the dynamics of ordering in low-temperature quenched nonstoichiometric A xB 1 − x binary alloys. By implementing the conserved spin-exchange dynamics into the Monte Carlo method the system evolves so that the density is preserved while the order parameter is not. The simulations have been carried out on a two-dimensional square lattice and the stoichiometric value of the composition x is x 0 =0.50. By using different values of x ranging from 0.60≤x≤ x 0 =0.50, we study the influence of the off-stoichiometry on the dynamics of ordering. Regarding the behavior of the excess particles all along the ordering process, we obtain two different regimes. (i) At early to intermediate times the density of excess particles at the interfaces rapidly increases, reaching a saturated value. This density of saturation depends on both composition and temperature. As a consequence of this, since the disorder tends to be localized at the interfaces, the local order inside the growing domains is higher than the equilibrium value. (ii) Once saturation is reached, the system evolves so that the density of excess particles at the interfaces remains constant. During this second regime the excess particles are expelled back to the bulk as the total interface length decreases. We use two different measures for the growth: the total interface length and the structure factor. We obtain that during the second regime scaling holds and the domain-growth process can be characterized, independently on x, by a unique length which evolves according to l(t)∼ t n being n ∼ (0.50–0.40). Although the growth process tends to be slower as x increases, we find that the domain-wall motion follows the main assumptions underlying the Allen-Cahn theory. This is indicative that the coupling between diffusive excess particles and curvature-driven interface motion does not modify the essential time dependence but varies (slows down) the growth rate of the growth law, i.e., l(t)= k 1 / 2 xt , with k x decreasing with x. We suggest that the logarithmic growth experimentally observed in some nonstoichiometric binary materials has to do with the existence of specific interactions (not present in our case) between diffusive particles and domain walls. These interactions are of crucial importance in determining the essential time dependence of the growth law

Monte Carlo study of the relation between vacancy diffusion and domain growth in two-dimensional binary alloys

Domain growth in a two-dimensional binary alloy is studied by means of Monte Carlo simulation of an ABV model. The dynamics consists of exchanges of particles with a small concentration of vacancies. The influence of changing the vacancy concentration and finite-size effects has been analyzed. Features of the vacancy diffusion during domain growth are also studied. The anomalous character of the diffusion due to its correlation with local order is responsible for the obtained fast-growth behavior